The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a pinned reference ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic (AFM) material. Alternatively, a synthetic antiferromagnet (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used to replace the ferromagnetic pinned layer.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, dictated by the minimum energy state, which is determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Although the layers enumerated above are all that is needed to produce the GMR or TMR effects, additional problems remain. In particular, there are certain noise effects associated with these structures. Magnetization in a layer can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem has been to provide a device structure conducive to ensuring that the free layer is a single domain so that the domain configuration remains unperturbed after fabrication and under normal operation.
FIG. 1 shows a typical prior art arrangement of the layers that make up an SV. In this example, the AFM and pinned layers are at the top so the device is referred to as a top spin valve (TSV). Seen there are substrate 11 over which lies free layer 16 on its seed layer 17. Pinned layer 14 lies on non-magnetic spacer layer 15 with AFM layer 13 immediately above it. A capping layer 12 completes the structure. It is important to note that, in all such devices of the prior art, the pinning layer contacts the pinned layer over its entire length and breadth.
In FIG. 2 we show an example of a bottom spin valve (BSV). Seen there are substrate 11 over which lies AFM layer 13 on its seed layer 18. Nonmagnetic spacer layer 15 lies on pinned layer 14 with free layer 16 immediately above it. Capping layer 12 completes the structure. As before, it is important to note that, in all such devices of the prior art, the pinning layer contacts the pinned layer over its entire length and breadth.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,636,397 B2, Gill discloses two structures featuring a split AFM layer but in both cases the split AFM layers flank a capping layer and the free layer. The pinned layer (200) is pinned by AFM layer 104 which fully underlies it in the standard way. In US 2003/0179507 Freitag et al. describe AFM layers in wing portions of a spin valve sensor. Their configuration is similar to Gill's, with pinned trilayer 220/222/224 being fully underlaid by AFM layer 216.